The present invention relates to a hydraulic system for a working machine.
The invention is applicable, according to an aspect thereof on working machines within the fields of industrial construction machines, in particular wheel loaders. Although the invention will be described with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other working machines such as articulated haulers, excavators and backhoe loaders.
A working machine is provided with a bucket, container or other type of implement for lifting, carrying and/or transporting a load.
For example, a wheel loader has a load arm unit for raising and lowering the implement, such as a bucket. The load arm unit comprises a number of hydraulic cylinders for movement of the load arm and the implement attached to the load arm. A pair of hydraulic cylinders is arranged for lifting the load arm and a further hydraulic cylinder is arranged on the load arm for tilting the implement. The wheel loader which usually is frame-steered has also a pair of hydraulic cylinders for turning/steering the wheel loader by pivoting a front part and a rear part of the wheel loader relative to each other.
In addition to the hydraulic cylinders, the hydraulic system of the wheel loader preferably comprise a first pump (working cylinder pump) for providing, hydraulic fluid to the hydraulic cylinders of the load arm unit and a second pump (steering cylinder pump) for providing hydraulic, fluid to the steering cylinders. In order to optimize the hydraulic system and use the pumps in an effective way, the steering cylinder pump can also be used for providing hydraulic fluid to the hydraulic cylinders (working cylinders) of the load arm unit.
For both economic and environmental reasons, the fuel efficiency of a working machine is increasingly important. One way of increasing the fuel efficiency in a working machine comprising hydraulic actuators s to increase the efficiency of the hydraulic system. It is for example known that fixed displacement pumps have a higher efficiency compared to variable pumps. However, only partial flows of the fixed displacement pumps are used, the remaining flow may have to be dumped to a tank, thereby incurring losses in the system.
U.S. Pat. No. 4,044,786 attempts to alleviate the aforementioned problem by introducing a hydraulic system where a priority valve is connected to a fixed displacement pump so that the flow of the pump is distributed to more than one load circuit.
However, in the hydraulic system according to U.S. Pat. No. 4,044,786, there will be a delay in response in the transition from the first pump to the second pump when the flow demand of the primary load circuit is increased beyond what the first pump can provide. Such a delay will be noticed by the operator of the vehicle as a temporary reduction of response of the primary load circuit.
In view of the above-mentioned desired properties of a hydraulic system, it is desirable to provide an improved hydraulic system for a work machine and a method for controlling such a system.
In the present context, a priority valve should be understood as a hydraulic valve having an inlet for receiving hydraulic fluid from a hydraulic machine and two outlets for directing the received hydraulic fluid to two hydraulic actuators. The priority valve is configured to be controlled to provide hydraulic fluid exclusively to either of the actuators, or to provide hydraulic fluid simultaneously to both actuators at any ratio there between based on a received control signal.
The present invention is, according to an aspect thereof, based on the realization that an efficient hydraulic system may be achieved by using a first and a second priority valve for providing hydraulic fluid to a first and a second actuator, where both priority valves are controlled by a pressure drop over an inlet valve of the first actuator, thereby efficiently utilizing two hydraulic machines while prioritizing the first actuator.
According to an aspect of the present invention, the first and the second priority valves are both connected to the first and the second hydraulic, actuators, and the priority valves are controlled by a pressure drop detected over the inlet, valve of the first (prioritized) actuator such that if the pressure drop is below a first threshold value, both priority valves provide hydraulic fluid only to the first actuator. If the pressure drop exceeds the first threshold value, the second priority valve starts to provide a flow of hydraulic fluid also to the second actuator, and if the pressure drop exceeds a second threshold value, the first priority valve starts to provide a flow of hydraulic fluid to the second actuator.
In one embodiment of an aspect of the invention, the second priority valve may further be configured to allow a flow of hydraulic fluid only to the second actuator for a pressure drop higher than a third threshold value, and the first priority valve may further be configured to allow a flow of hydraulic fluid only to the second actuator for a pressure drop higher than a fourth threshold value, wherein the fourth threshold value is higher than the third threshold value. Furthermore, the third threshold value may advantageously be higher than the second threshold value. Thereby, the two priority valves have ranges which are overlapping when a change in pressure drop causes a transition from providing hydraulic fluid exclusively to either of the actuators to providing hydraulic fluid to both of the actuators, and vice versa. By configuring the priority valves such that the threshold values overlap, a smooth transition between the first actuator and the second actuator can be achieved.
According to one embodiment of an aspect of the invention, the first function may be a hydraulic steering system of the working machine and the second function may be a hydraulic work system of the working machine. Thus, the first actuator is an actuator for providing steering function and the second actuator is an actuator for providing a working function, such as for example a lift function, of the working machine. Consequently, in one embodiment of the invention, the steering function is the prioritized function of the hydraulic system. Thereby, the hydraulic system is arranged to provide power to the lift actuator as long as no or low power is required by the steering actuator, and to redirect power to the steering function when so required.
In one embodiment of the invention, the first hydraulic machine may advantageously be a first fixed displacement hydraulic pump and the second hydraulic machine may advantageously be a second fixed displacement hydraulic pump.
A fixed displacement pump is a hydraulic pump which cannot be adjusted to increase or decrease the amount of fluid that is moved in one pump cycle. By using fixed displacement pumps instead of variable displacement pumps a simpler, more robust, and relatively low cost system can be provided.
In one embodiment of an aspect of the invention, the second fixed displacement hydraulic pump may advantageously have a larger displacement than the first fixed displacement hydraulic pump. Thereby, the steering may be powered by the first pump having a smaller fixed displacement as long as the steering load is sufficiently low, for example for low steering speeds such as during transportation of the working machine.
According to one embodiment of an aspect of the invention, the first priority valve run comprise a first spring and the second priority valve may comprise a second spring, wherein a position of each of the springs controls the output of the corresponding priority valve such that when the spring is fully extended, a flow of hydraulic fluid is allowed only to the first actuator, and when the spring is fully compressed, a flow of hydraulic fluid is allowed only to the second actuator.
Furthermore, the compression of the first spring and the second spring is preferably controlled by the pressure drop over the inlet valve of the first actuator. Moreover, the first spring may be configured to be fully extended for a pressure drop lower than the second threshold value and fully compressed for a pressure drop higher than the fourth threshold value. The second spring can be configured to be fully extended for a pressure drop lower than the first threshold value and to be fully compressed for a pressure drop higher than the third threshold value.
According to a second aspect of the invention, there is also provided a method for controlling a hydraulic system comprising a first hydraulic machine for providing hydraulic fluid to at least one of a first actuator and a second actuator via a first priority valve and a second hydraulic machine for providing hydraulic fluid to at least one of the first actuator and the second actuator via a second priority valve, the first hydraulic actuator being provided with an inlet valve, the method comprising: controlling each of the first priority valve and the second priority valve to allow a flow of hydraulic fluid only to the first actuator if a pressure drop over the inlet valve of the first actuator is below a first threshold value; controlling the second priority valve to allow a flow of hydraulic fluid to both of the first actuator and the second actuator for a pressure drop higher than the first threshold value; controlling the first priority valve to allow a flow of hydraulic fluid to both of the first actuator and the second actuator for a pressure drop higher than a second threshold value; wherein the second threshold value is higher than the first threshold value.
Effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention.
There is also provided a computer program for causing a processing unit to control a hydraulic system of a working machine according to the abovementioned method for controlling a hydraulic system, and a computer readable medium comprising said computer program.
Further features of and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.